This invention is directed to radiography. In particular, it is directed to an asymmetric radiographic silver halide film that provides improved medical diagnostic images of soft tissues such as in mammography.
The use of radiation-sensitive silver halide emulsions for medical diagnostic imaging can be traced to Roentgen""s discovery of X-radiation by the inadvertent exposure of a silver halide film. Eastman Kodak Company then introduced its first product specifically intended to be exposed by X-radiation in 1913.
In conventional medical diagnostic imaging the object is to obtain an image of a patient""s internal anatomy with as little X-radiation exposure as possible. The fastest imaging speeds are realized by mounting a dual-coated radiographic element between a pair of fluorescent intensifying screens for imagewise exposure. About 5% or less of the exposing X-radiation passing through the patient is adsorbed directly by the latent image forming silver halide emulsion layers within the dual-coated radiographic element. Most of the X-radiation that participates in image formation is absorbed by phosphor particles within the fluorescent screens. This stimulates light emission that is more readily absorbed by the silver halide emulsion layers of the radiographic element.
Examples of radiographic element constructions for medical diagnostic purposes are provided by U.S. Pat. No. 4,425,425 (Abbott et al.) and U.S. Pat. No. 4,425,426 (Abbott et al.), U.S. Pat. No. 4,414,310 (Dickerson), U.S. Pat. No. 4,803,150 (Kelly et al.), U.S. Pat. No. 4,900,652 (Kelly et al.), U.S. Pat. No. 5,252,442 (Tsaur et al.), and Research Disclosure, Vol. 184, Aug. 1979, Item 18431.
While the necessity of limiting patient exposure to high levels of X-radiation was quickly appreciated, the question of patient exposure to even low levels of X-radiation emerged gradually. The separate development of soft tissue radiography, which requires much lower levels of X-radiation, can be illustrated by mammography. The first intensifying screen-film combination (imaging assembly) for mammography was introduced to the public in the early 1970""s. Mammography film generally contains a single silver halide emulsion layer and is exposed by a single intensifying screen, usually interposed between the film and the source of X-radiation. Mammography utilizes low energy X-radiation, that is radiation predominantly of an energy level less than 40 keV.
U.S. Pat. No. 6,033,840 (Dickerson) and U.S. Pat. No. 6,037,112 (Dickerson) describe asymmetric imaging elements and processing methods for imaging soft tissue.
Problem to be Solved
In mammography, as in many forms of soft tissue radiography, pathological features that are to be identified are often quite small and not much different in density than surrounding healthy tissue. Thus, relatively high average contrast, in the range of from 2.5 to 3.5, over a density range of from 0.25 to 2.0 is typical. Limiting X-radiation energy levels increases the absorption of the X-radiation by the intensifying screen and minimizes X-radiation exposure of the film, which can contribute to loss of image sharpness and contrast. Thus, mammography is a very difficult task in medical radiography. In addition, microcalcifications must be seen when they are as small as possible to improve early detection and treatment of breast cancers. As a result, there is desire to improve the image quality of mammography films by increasing image sharpness.
This invention provides an improved radiographic silver halide film comprising a support having first and second major surfaces and that is capable of transmitting X-radiation,
the radiographic silver halide film having disposed on the first major support surface, two or more hydrophilic colloid layers including first and second silver halide emulsion layers with the second silver halide emulsion layer being closer to the support and further comprising a crossover control agent, and having disposed on the second major support surface, two or more hydrophilic colloid layers including a third silver halide emulsion layer and an antihalation layer disposed over the third silver halide emulsion layer,
each of the first and second silver halide emulsion layers comprising cubic silver halide grains that have the same or different composition in each silver halide emulsion layer, and the third silver halide emulsion layer comprising tabular silver halide grains,
the crossover control agent being present in an amount sufficient to reduce crossover to less than 10%, and is substantially removed from the film during wet processing within 90 seconds.
This invention also provides a radiographic imaging assembly comprising a radiographic silver halide film of this invention that is arranged in association with a fluorescent intensifying screen.
Further, this invention provides a method of providing a black-and-white image comprising exposing a radiographic silver halide film of this invention and processing it, sequentially, with a black-and-white developing composition and a fixing composition, the processing being carried out within 90 seconds, dry-to-dry.
The present invention provides a means for providing radiographic images for mammography exhibiting improved image quality by providing images of improved sharpness due to reduced crossover (for example, less than 10%) of light transmitted through the support to the backside silver halide emulsion when the film is exposed using a single fluorescent intensifying screen on one side of the film (the frontside).
In addition, all other desirable sensitometric properties are maintained and the asymmetric radiographic film can be rapidly processed in the same conventional processing equipment and compositions.
These advantages are achieved by using a novel combination of emulsion layers in the radiographic film. On the frontside are two cubic grain emulsions, the emulsion layer closer to the support also comprising crossover control agent to reduce crossover through the support. The single backside emulsion includes tabular silver halide grains, and an antihalation layer is disposed over the backside emulsion layer. In addition, the two cubic grain emulsion layers are different in thickness with the emulsion layer closer to the support being thinner than the other cubic grain emulsion layer and containing a crossover control agent.